This invention relates to an electromagnetic shield for an electromagnetic apparatus, such as a transformer, reactor or current transformer, to allow operation of the apparatus with high degree of accuracy by preventing any influences of external magnetic flux.
FIG. 1 is a cross sectional view of a current transformer 55 with a conventional electromagnetic shield 53, as disclosed in Japanese Utility Model Laid Open No. 58-124931.
Referring to FIG. 1, the current transformer 55 has two ring-shaped magnetic cores 48, 49 with windings 50, 51 wound thereon, respectively. The electromagnetic shield 53 which consists of electrical conductors covers the cores 48, 49 and the windings 50, 51 separated by an insulator 52. An insulator 54 is mounted externally of the electromagnetic shield 53 to prevent the shield 53 acting as one-turn short circuit inter-linked with main flux paths in the cross 48, 49. The electromagnetic shield 53 is molded with resin containing electrically conductive powder of copper or aluminum, or is made of a conductive plate of copper or aluminum.
In the current transformer 55 constituted above, the electromagnetic shield 53 does not affect the main flux flowing in the cores 48, 49. When some external magnetic flux acts on the current transformer 55, the electromagnetic shield 53 permits eddy currents to flow therein creating a magnetic field opposing the external magnetic flow fluxes and preventing any influence of the external magnetic fluxes on the windings 50, 51.
Accordingly, one is able to carry out the current measurement with sufficient accuracy without any influences of the external magnetic fluxes in such a current transformer.
In such a conventional electromagnetic shield, there are several disadvantages. When the molded resin containing electrically conductive powder is used as the electromagnetic shield 53, its performance goes down at the higher frequency range because of the specific resistance of the resin. And in the case where the shield 53 is of copper or aluminum plate, its manufacture is difficult and its cost is high because it requires making separate types of casts for each type of electromagnetic shield using special tools.